Extremely reliable, go and come back (6)
I believe the answer is:
return
'come back' is the definition.
('return' can be a synonym of 'come back')
'extremely reliable go' is the wordplay.
'extremely' suggests removing the centre.
'go' becomes 'turn' ('turn' can be a synonym of 'go'**).
'reliable' with its middle taken out is 're'.
're'+'turn'='RETURN'
'and' is the link.
(Other definitions for return that I've seen before include "answer" , "Come back; profit" , "Yield" , "Two-way ticket" , "Interest" .)